Please refer to FIG. 1 that is a diagram of a conventional Buck converter for driving light-emitting diode (LED). When the transistor switch Q2 is turned on, current is supplied from the power source V4 and flows through the transistor switch Q2 to store energy in the inductor L1 and the capacitor C, meanwhile the current supplies energy to LEDs D2, D5 for the same to emit light. When the transistor switch Q2 is turned off, energy is released from the inductor L1 and the capacitor C4 to the LEDs D2, D5 for the same to emit light continuously. The current flowed through the LEDs D2, D5 causes a voltage drop at two ends of the resistor R7 to thereby generate a current detection signal. The error amplifier U4 receives at the negative input thereof the current detection signal and at the positive input thereof a voltage reference signal Vref, and outputs an error amplification signal at the output thereof according to the received signals. The pulse width modulation (PWM) comparator U5 receives the error amplification signal and a triangular wave signal, compares the two signals, and outputs a PWM signal to determine the turn-on duration of the transistor switch Q2 in each cycle, so as to control the amount of energy supplied by the power source V4 to the buck converter. Basically, the turn-on duration of the transistor switch Q2 is controlled by a feedback voltage to generate sufficient voltage and current, so that a voltage across the resistor R7 is the same as a preset value. In the event the output current is exceeded or insufficient, the voltage across the resistor R7 will change, which in turn changes the turn-on duration of the transistor switch Q2 for keeping the output current at a stable current.
Please refer to FIG. 1 again. The buck converter for driving LED also uses a NAND gate U7A to simultaneously receive a dimming signal and the PWM signal, so as to achieve the dimming function. When the input dimming signal is low level, the output at the NAND gate U7A turns to high level to turning off the power transistor Q2. When the transistor Q2 is turned off, the inductor L1 and the capacitor C4 release energy via the diode D8 to keep the LEDs D2, D5 emitting light. However, since the dimming signal has a relatively long period, the inductor L1 and the capacitor C4 will keep releasing energy until the output voltage (that is, the voltage at the connection point of the inductor L1 and the capacitor C4) is lower than the threshold voltage of the LEDs D2, D5, and then the LEDs D2, D5 no longer emit light. However, since there are some current leakage paths, the output voltage will keep dropping until all the stored energy in the inductor L1 and the capacitor C4 is completely released. When the dimming signal turns to high level, the output at the NAND gate U7A turns to be controlled by the PWM signal and the previously described feedback control is resumed to obtain stable light emission. In this manner, the ratio of the turn-on duration to the turn-off duration of LED is controlled by changing the pulse width of the pulse signal to thereby obtain an average brightness and achieve the purpose of dimming.
Since the above-described dimming operates at two extremities of turn-on and turn-off, an overly high transient voltage will be generated. FIG. 2 shows the operating voltage and current waveforms of the above conventional LED driving circuit. As shown, the voltage varies between zero volt and N*VF volt, where N is the number of LEDs being connected in series; VF is the forward bias voltage of an LED with a predetermined current flowed through the LED; and N*VFmin is the threshold voltage of. If there are several tens of LEDs connected in series, the voltage difference between the two ends of the LEDs shall be as high as several hundreds of volts. And, when the output voltage is dropped from N*VF to N*VFmin, the LEDs almost stop emitting light, as shown in FIG. 2. At this point, the capacitor releases the stored energy at a gradually slowed speed until all the stored energy is completely released. The LEDs require time to be turned on again. Under this circumstance, when an excessively high voltage is instantaneously supplied to the LEDs, some of the LEDs would with stand a voltage stress much higher than that as stipulated in the specification thereof when they are differently turned on. This condition surely would dangerously cause burnout of these LEDs. Therefore, the above-described conventional LED driving circuit must be provided with a voltage ramp-up control circuit to solve the above-mentioned problems. However, the provision of the voltage ramp-up control circuit not only increases the cost of the LED driving circuit, but also increases the transient time of voltage variation during the dimming operation to thereby adversely affect the accuracy in dimming control.